The field of the disclosure relates generally to sealing surface discontinuities, and, more particularly, to seals for encapsulating gaps, edges, ledges, and other discontinuities on an aircraft structure.
Many structures, such as aircraft structures, include a plurality of assemblies that may create gaps, edges, ledges, and other discontinuities where elements of the assemblies interface. Efficient and safe operation of an aircraft, for example, requires that such discontinuities be sealed. Traditionally, such discontinuities are sealed by the direct application of wet sealant using a manually operated extrusion device and hand tools. However, such direct application of wet sealant poses several difficulties. Typically, other work must be delayed for 48 to 72 hours near the area of the structure to which wet sealant has been applied, to enable the wet sealant to cure sufficiently to avoid contamination or damage impacts from other work. Moreover, in some circumstances, air may become entrained as the wet sealant is applied, or the manual application of wet sealant may involve short pauses and restarts, each of which tends to create bubbles and voids within the applied sealant. Furthermore, it may be necessary to apply successive layers, or “beads,” of wet sealant in a stacked fashion to achieve the desired thickness of the seal, and voids tend to occur between each layer.
In addition, the quality of application of the wet sealant may be sensitive to temperature. Wet sealant material that is colder than an optimal temperature tends to be too thick, and thus may not flow sufficiently to cover the discontinuity as intended. On the other hand, wet sealant material that is hotter than the optimal temperature may partially cure during application, sometimes referred to as “cross-linking” of the wet sealant. Cross-linking in the wet sealant also limits the ability of the sealant to flow smoothly to cover the discontinuity as intended, and to be worked with hand tools into a desired configuration immediately after application. In each case, streams or strands of sealant may separate and re-enter the seal region, trapping air inside and/or failing to integrate fully with the seal. Often it may be difficult to precisely control the temperature of the wet sealant throughout an application.
Seal regions that have an undesirable number of bubbles and voids must be reworked, and typically the rework may be performed only after the originally applied sealant has cured for 48 to 72 hours, for the reasons described above. Furthermore, the reworked portions of the seal typically must be allowed to cure for an additional 24 to 48 hours. In addition, pieces of re-entrant sealant may chip off during rework, creating a risk of foreign object debris for the rework.
Moreover, in some circumstances, an excess of wet sealant is applied to ensure an acceptable performance of the seal. Such excess sealant can add significant unnecessary weight to a structure such as an aircraft, adversely affecting the efficiency of operation. In addition, direct application of wet sealant can create irregular outer edges of the seal that are visually unappealing to customers. Thus, in some circumstances, direct application of wet sealant to structural discontinuities causes extended delays and increased expense in both manufacture and operation. The individuals who manually apply the wet sealant may need extended training and years of experience in order to successfully avoid the drawbacks described.
Some known seals use a cap or mold to control application of wet sealant directly to a structure, such as an aircraft. However, in at least some cases, the use of such a cap or mold does not prevent the entrainment of air during application of the wet sealant. In addition, the use of a cap or mold at the structure does not avoid the need to delay other work to enable the wet sealant to cure sufficiently to avoid contamination or damage impacts.
Some known seals are molded or extruded into a desired shape prior to installation on a structure, such as an aircraft. However, at least some known molding and extruding techniques also create defects in the seal. For example, during the injection of wet sealant into at least some known molds, the viscous wet sealant flowing against the interior surfaces of the mold tends to stack up on itself, producing folds along the edges of the seal. In addition, during extrusion of wet sealant through at least some known extrusion dies, the viscous sealant material tends to curl back toward the edges of the extrusion die, which deforms the intended cross-sectional shape of the seal.